Ah, this is something I can actually answer and shed light on, as I’ve read quite a few tech articles about it.
Fair warning, I’m going to get technical here, but I will at least do my best to attempt to explain all of it in such a way that it makes sense to the non-technical.
To get an understanding on why Blizzard is Blizzard, and what sets them so far ahead of other MMO creators is actually the core component. If you’re hoping I’m going to talk about game design philosophy here, you should probably leave now, cause we’re totally going in the opposite direction. It’s a long read, but does go into a lot of detail…
I somehow doubt WoW is anywhere near the size of google. Google is estimated to have somewhere around a million servers. Microsoft has a million as well, Facebook, and Amazon probably both run somewhere north of 200,000 by all estimations. And this isn’t speaking about places like Walmart who processes something like 35 million customer transactions daily.
My estimation for blizzard is a lot more conservative:
There’s 246 US realms. The last time blizzard gave out numbers there were 4 servers per realm: 2 for the world servers, one for chat, and one for instances. 246 * 4 = 984. Even if we assume that those two were only EK/Kalimdor and we add physical machines for Outland/Northrend/Pandaria that only brings us to a total of 1700 machines for realms alone.
World of Warcraft at its peak, in 2010, had 10 million subscribers. Which is easily handed by your typical SQL database. eBay, for example, was handling 212 million users and 26 billion SQL transactions a day on an Oracle cluster. Facebook is doing crazy things with MySQL to handle 800 million users and and 60 million queries per second. And if you check out Blzzard’s hiring requirements for DBAs you’ll actually see that they want people with Oracle experience.
Blizzard’s core competency is games. I doubt they’d go in the same direction as facebook or google or have problems so large that they’d make their own database backend. Most of the game state doesn’t actually have to be stored in the backend.
On the other hand, software development is hell. I could easily see people coming from Titan needing this much time to come up to speed. Especially given the age of the WoW codebase (they started writing this 14 years ago.)